When an element is heated or otherwise energetically excited, it emits radiation at a set of wavelengths or spectral lines which are characteristic of that particular element, and is termed its emission spectrum. A particular emission spectrum is the signature of a particular element. A material, which is composed of a number of elements and which is heated or otherwise energetically excited, will produce an emission spectrum which is equal to the sum of the emission spectra of all the elements of which the material is composed and one can ascertain the elemental composition of a material by determining the material's emission spectrum.
An emission spectrometer excites a material and then separates (disperses) radiation emitted from the material into the spectra of the elements which make up the material. Radiation can be dispersed by various means including refraction and diffraction. Modern spectrometers typically use a diffraction grating as the dispersive element to separate the various component wavelengths of light. A diffraction grating typically comprises a reflective or transmissive substrate in which a series of grooves have been formed or a surface onto which a series of holographic fringes have been recorded. In either case, the fringes or the grooves are regularly spaced and cause radiation of different wavelengths to destructively or constructively interfere as a function of the wavelength of radiation, the separation of the grooves or fringes and the angle at which the radiation impinges upon and is reflected from the grooves or fringes. The relationship which determines which wavelengths are selected to undergo constructive interference is termed the grating equation and is given by: EQU n.lambda.=d SIN .THETA..+-.d SIN .THETA.'
where n is the order, which is equal to one for the fundamental order, .lambda. is the wavelength; d is the spacing between the grooves or fringes, .THETA. is the angle at which the light is incident upon the grating and .THETA.' is the angle at which the light is diffracted from the grating. By knowing the groove spacing and the incident angle, one can calculate the wavelength which is constructively diffracted by the grating at a particular angle.
Although a set of wavelengths uniquely defines a given element, a particular spectral line may be produced by different elements. Further, several elements within a material may produce spectral lines which, although are not at the same wavelength, may be close to one another. The ability to separate two or more closely spaced spectral lines is termed the resolving power of the spectrometer.
Although diffractive dispersion elements have attributes which are shared by refractive dispersive elements, such as prisms, diffractive elements also possess a property not shared by refractive elements, that is, they produce different harmonics or integer multiples (n&gt;1) of the grating fundamental (n=1) wavelength satisfying the grating equation. This property results in the constructive interference of wavelengths which are multiples, termed higher orders, of the fundamental wavelength, as well as the fundamental wavelength. When it is desired to measure the intensity of a secondary wavelength, a fundamental line of strong intensity may obscure a higher order line, thereby preventing its measurement.
To separate the fundamental wavelength from other orders, bandpass filters have been used to allow only those wavelengths of light which fall within a certain band of wavelengths, as determined by the filter, to pass. These filters typically reject or prevent most of the light of wavelengths outside the pass band; allowing only 10.sup.-3 to 10.sup.-4 of the intensity incident upon the filter to pass through. Such filters are usually classed as wideband or narrowband filters, depending upon the range of wavelengths the filter allows through. Wideband filters typically pass a wide range of wavelengths but are generally not available for filtering below 300 nanometers. Narrowband filters typically pass a narrow range of wavelengths, for example a range of 10-20 nanometers, and are available for filtering below 300 nanometers, but are expensive and easily damaged. A second known method for separating fundamental and higher order wavelengths is the use of a "solar blind" photomultiplier tube. While this type of sensor is capable of rejection of the order of 10.sup.-4 to 10.sup.-5, it is expensive.